DNV-RP-F101 Corroded Pipe Calculator (Single Defect Capacity)

Returns the length correction factor Q = √(1 + 0.31·(ℓ/√(D·t))²), the burst capacity pressure P_cap of a single longitudinal metal-loss defect, the safe working pressure P_sw = usage factor · P_cap, and the utilization against your operating pressure — the DNV-RP-F101 Part B capacity form. The tensile strength f_u and the combined usage/safety factor stay user inputs.

Method last updated 2026-07-30 (calculation changelog) · fixture-verified on every build — most recently 2026-07-31.

DNV-RP-F101 is the other standard answer to the question B31G answers: how much pressure can a corroded pipe still carry? The two methods reach that answer differently, and knowing which one you are running matters more than most engineers assume, because they can disagree by a wide margin on the same defect.

B31G — original and Modified alike — is built on a flow stress derived from yield strength and a Folias bulging factor, and it was calibrated against a body of full-scale tests on older, lower-grade line pipe. RP-F101's capacity form is built on tensile strength f_u and a different length correction Q, and it was calibrated on a later and broader test population including modern higher-grade steels. The practical consequence is that RP-F101 is generally less conservative on long, shallow defects, and the difference is not a rounding error: it is common for a defect that B31G rejects to pass an RP-F101 assessment, and that difference is frequently the whole justification for keeping a line in service rather than cutting it out.

The other structural difference is where the safety lives. B31G folds its conservatism into the method — the parabolic or 0.85 area idealisation, the flow-stress definition — and then applies a safety factor on top. RP-F101 separates the two explicitly: the capacity equation returns a best-estimate burst pressure, and everything protective is applied afterwards through the usage factor. That is why this card takes the usage factor as an input and lets you set it to 1.0: entering 1.0 reads the raw capacity equation, which is what you want when you are comparing methods or checking somebody else's assessment, and it is not what you want when you are dispositioning a defect.

What the usage factor should be is genuinely not a number this tool can supply. In RP-F101 practice it is assembled from the assessment method being used (allowable-stress versus partial-safety-factor), the design usage factor for the line's location class, and the confidence in the inspection data — depth measurement accuracy and its standard deviation feed the partial-safety-factor route directly. That combination is the assessment engineer's, taken from the governing recommended practice and the inspection's demonstrated tolerance.

Method

The single-defect capacity form for a longitudinal metal-loss defect under internal pressure. First the length correction, which converts the defect's slenderness relative to the pipe's characteristic length √(D·t) into a capacity reduction:

Q = √( 1 + 0.31 · ( ℓ / √( D · t ) )² )

Then the capacity (best-estimate burst) pressure of the defected section:

P_cap = ( 2 · t · f_u / ( D − t ) ) · ( 1 − d/t ) / ( 1 − (d/t) / Q )

and the safe working pressure and utilization:

P_sw = usage factor · P_cap

utilization = P_op / P_sw → PASS when utilization ≤ 1

Read the capacity equation as three factors. The leading term 2·t·f_u/(D−t) is the burst pressure of sound pipe in the mean-diameter form — note the (D−t) denominator rather than D, which is not the Barlow thin-wall form and gives a slightly higher number. The (1 − d/t) numerator is the fraction of wall the defect leaves behind. The (1 − (d/t)/Q) denominator is where length enters: a very short defect has Q ≈ 1, the two terms nearly cancel, and the capacity approaches sound pipe; a long defect has a large Q, the denominator approaches 1, and the capacity collapses toward the plain (1 − d/t) fraction of the sound-pipe burst — the infinitely-long-defect limit, where the remaining ligament simply carries the hoop load on its own.

Inputs are validated rather than trusted: D and t positive with t < D/2, the defect depth strictly less than the wall (a through-wall defect is a leak, not an assessment), f_u and the usage factor positive, and the operating pressure non-negative. Entering P_op = 0 puts the card in capacity-only mode: it returns P_cap and P_sw and suppresses the verdict, which is what you want when you are establishing what a defect can carry rather than checking it against a known pressure. A standing warning fires above d/t = 0.85, outside the validated range of the single-defect equation, and every run is flagged as applying to an isolated defect only.

Limitations — what this calculator is not

• Single isolated defect only. Real corrosion arrives in colonies, and defects that are close enough to interact carry a combined capacity lower than any of them individually. Assessing interaction — and the axial and circumferential spacing rules that decide when two defects must be treated as one — requires the full RP-F101 procedure, not this equation. Running each pit separately through this card and taking the worst is a known way to get an unconservative answer.
• Longitudinal defects under internal pressure. Circumferentially-oriented metal loss, defects in bends and fittings, and any case where axial load or bending is significant are outside this form. RP-F101 has provisions for combined loading; they are not implemented here.
• Blunt metal loss. Corrosion pitting and general wall loss are what the equation describes. Cracks, crack-like features, gouges, laminations and mechanical damage are not metal loss in this sense and can fail well below the capacity this returns.
• f_u is your input and the answer scales linearly with it. Which tensile strength to use — specified minimum, actual from the material certificate, or a derated value — is set by the assessment route and the governing recommended practice, and it is not a free choice. An assessment run on actual mill tensile values and one run on specified minimum are different assessments with different acceptance criteria.
• The usage factor carries all the safety and this tool does not select it. In RP-F101 it is assembled from the assessment method, the location-class design usage factor and the inspection accuracy — including the depth measurement's standard deviation in the partial-safety-factor route. Entering 1.0 gives the raw capacity equation with no protection applied at all; that is a deliberate diagnostic mode, not a disposition.
• Measurement uncertainty is not modelled. The depth you enter is treated as exact. Real ILI depth calls carry a tolerance and a confidence level, and the RP-F101 partial-safety-factor route exists specifically to handle that — this card applies whatever single depth value you give it.
• Above d/t = 0.85 the equation is outside its validated range and the engine says so, but it still returns a number. Deep defects are normally repaired rather than assessed.
• It returns a pressure, not a disposition or a remaining life. Corrosion grows; a defect that passes today has a future, and setting a re-inspection interval requires a corrosion rate and a growth assessment that this calculation does not perform.

Quick reference — how defect length changes the answer

The length correction Q is the part of this method engineers find least intuitive, so it is worth seeing what it does across the range. Values below are the equation evaluated for the worked example's NPS 12 × 0.375 in geometry, where √(D·t) = 2.18661 in and d/t = 0.4:

The engineering consequence: for short defects, an error in the length measurement barely moves the capacity, while an error in depth moves it a lot. For long defects the reverse is not true — length has saturated, and depth is again the sensitive input. Depth accuracy is what deserves the inspection budget in both regimes.

Worked example — fixture-verified

An NPS 12 line, 12.75 in OD × 0.375 in wall in API 5L X52 material, with the assessment run on a tensile strength of 66,700 psi. In-line inspection reports an isolated longitudinal metal-loss defect 6 in long with a maximum depth of 0.15 in — 40% of the wall. The assessment applies a combined usage factor of 0.72, and the line operates at 1,440 psi.

Step by step

1. Characteristic length: √(D·t) = √(12.75 · 0.375) = √4.78125 = 2.18661 in.
2. Normalised defect length: ℓ / √(D·t) = 6 / 2.18661 = 2.74398.
3. Length correction: Q = √(1 + 0.31 · 2.74398²) = √(1 + 0.31 · 7.52941) = √3.33412 = 1.82596.
4. Depth ratio: d/t = 0.15 / 0.375 = 0.4.
5. Sound-pipe term: 2·t·f_u / (D − t) = 2 · 0.375 · 66,700 / (12.75 − 0.375) = 50,025 / 12.375 = 4,042.424 psi.
6. Defect factor: (1 − 0.4) / (1 − 0.4/1.82596) = 0.6 / (1 − 0.21906) = 0.6 / 0.78094 = 0.76831.
7. Capacity pressure: P_cap = 4,042.424 · 0.76831 = 3,105.826 psi.
8. Safe working pressure: P_sw = 0.72 · 3,105.826 = 2,236.194 psi.
9. Utilization: 1,440 / 2,236.194 = 0.64395 → the line runs at about 64% of the defect's safe working pressure, so the verdict is PASS.

A comfortable pass with a caveat worth stating. The defect factor of 0.768 means this 40%-deep, 6-inch defect has cost the pipe about 23% of its burst capacity — noticeably more than the 40% depth alone would suggest is left, because the length correction is doing real work at ℓ/√(Dt) = 2.74. The 0.64 utilization is against a safe working pressure that already carries the 0.72 usage factor, so the margin to best-estimate burst is much larger than it looks: 1,440 psi against 3,105.8 psi is a factor of 2.16. What this result does not include is time. Corrosion that produced a 0.15 in defect will keep going, and the number that decides the re-inspection interval is the growth rate, not this utilization.

Worked example 2 — capacity-only mode, reading the raw equation

The same pipe and the same defect, run with the usage factor set to 1.0 and no operating pressure entered. This is the diagnostic mode: it strips every protective factor out and reports what the capacity equation alone says the defected section can carry.

Step by step

1. Q, the depth ratio and the capacity equation are unchanged — none of them depends on the usage factor or on operating pressure: P_cap = 3,105.826 psi.
2. With the usage factor at 1.0, the safe working pressure equals the capacity: P_sw = 1.0 · 3,105.826 = 3,105.826 psi.
3. No operating pressure was entered, so utilization is suppressed and the card returns no verdict — status is COMPUTED rather than PASS or FAIL.
4. Comparing the two runs: the 0.72 usage factor in the main example is discarding 869.6 psi, or 28% of the best-estimate capacity. That is the entire safety margin of the assessment, visible as a single number.

Capacity-only mode exists for three jobs, and dispositioning a defect is not one of them. It lets you compare methods on a like-for-like basis — RP-F101's raw capacity against a B31G failure pressure, both before any safety factor. It lets you check somebody else's assessment by separating the equation from the factors they chose. And it lets you see how much of your answer is physics and how much is conservatism, which is the question that comes up whenever a defect is close to the line and somebody asks whether the assessment is being unfair to it. Never quote a capacity-only number as a safe pressure.

Fixture case “capacity-only mode (Pop=0)” (tolerance 0.05) — locked in the same release gate as the example above.

Sources & citations

• DNV-RP-F101, Corroded Pipelines — Part B allowable-stress capacity form for a single longitudinal metal-loss defect under internal pressure, including the length correction factor Q. Usage/safety factors and the tensile strength basis are user-supplied from the governing revision.
• ASME B31G, Manual for Determining the Remaining Strength of Corroded Pipelines — the alternative method, built on a yield-derived flow stress and the Folias factor; see the B31G calculator for the same defect assessed that way.
• API 5L, Line Pipe — source of the specified minimum tensile strength; whether the assessment uses specified minimum or actual certified values is set by the assessment route, not by this tool.
• ASME B31.8S, Managing System Integrity of Gas Pipelines — where a remaining-strength assessment sits within an integrity management programme, and the basis for re-inspection intervals that this calculation does not set.
• No usage factor, safety factor or material value is embedded — all are entered by the user.

Per the source & citation policy, allowable-stress and factor table values are user-supplied. Where a page does reproduce specific ASME data (the B16.5 ratings, the quick-reference tables), it does so under ASME authorization and states the source table and conditions inline.

FAQ

RP-F101 or B31G — which should I use?

Whichever your integrity procedure and the governing regulation specify; this is not usually a free choice. Where it is, the practical differences are these. B31G (original and Modified) is built on a yield-derived flow stress and the Folias bulging factor, is embedded in US pipeline regulation and long-standing operator practice, and is generally the more conservative of the two — markedly so on long, shallow defects. RP-F101 is built on tensile strength with a different length correction, was calibrated on a broader and more modern test population including higher grades, and separates the best-estimate capacity from the safety factors applied to it. Running both on the same defect is a reasonable diagnostic; picking whichever one gives the answer you want after the fact is not.

What should I enter for the usage factor?

Not a default, because there is no universal one. In RP-F101 the factor applied to capacity is assembled from the assessment route you are using — allowable-stress or the partial-safety-factor method — the design usage factor for the line's location class and fluid, and, in the partial-safety-factor route, the accuracy of the inspection that measured the defect, including the standard deviation of the depth call. A poorly characterised defect legitimately gets a harsher factor than a well characterised one, and that is a feature of the method rather than a nuisance. Take the combination from the governing revision of the recommended practice. If you enter 1.0, you are reading the raw capacity equation and nothing is protecting you.

Why does the sound-pipe term use (D − t) instead of D?

Because it is the mean-diameter form of the burst expression rather than the thin-wall Barlow form. Barlow uses the outside diameter and is the conventional design equation; the mean-diameter form is closer to the actual stress distribution through the wall and is what the RP-F101 capacity equation was calibrated with. For an NPS 12 × 0.375 in pipe the difference is about 3% — 4,042 psi versus 3,923 psi on the leading term — which is small but not negligible when you are comparing this method's output against a B31G number computed on D. If you are cross-checking the two methods by hand, this is one of the two places the arithmetic will refuse to line up; the flow-stress definition is the other.

My corrosion is a cluster of pits, not one defect. Can I use this?

Not directly, and this is the most consequential limitation on the page. Defects that lie close enough to each other interact: the combined feature behaves as one larger defect with a lower capacity than any of its parts assessed alone. RP-F101 has explicit interaction rules — axial and circumferential spacing criteria that decide when adjacent metal loss must be combined, and a procedure for assessing the composite profile — and none of that is implemented here. Assessing each pit separately and taking the worst result will produce a number that is too high. Use the full procedure, or a river-bottom profile assessment, for anything that is not a genuinely isolated defect.

Does a passing result mean I can leave the defect alone?

It means the defect meets the acceptance criterion at the pressure and with the factors you entered, today. Two things it does not tell you. It does not set a re-inspection interval: corrosion grows, and the interval comes from the growth rate and the margin you have, which means a defect passing at 0.64 utilization and one passing at 0.98 need very different responses even though both say PASS. And it says nothing about the mechanism — a defect under active corrosion with a failed coating and no cathodic protection is a different proposition from a dormant one, whatever the current utilization is. Remaining strength is an input to an integrity decision, not the decision.

Can I use this on a defect in a bend, a fitting or a weld?

No. The capacity form assumes straight pipe, an isolated longitudinal defect in the pipe body, and internal pressure as the only significant load. Bends and fittings have different geometry and stress distributions; welds have different material properties and their own acceptance rules; and any location where axial load or bending is significant needs the combined-loading provisions of the full recommended practice rather than this equation. Metal loss found in those locations is normally taken to a fitness-for-service assessment with the actual geometry and loading modelled, not screened with a pipe-body equation.

Related calculators

• B31G Corroded Pipe Remaining Strength Calculator (Level 1) — The yield-based alternative on the same defect — usually the more conservative
• Pipeline MAOP Calculator — Maximum Allowable Operating Pressure (ASME B31.8 / B31.4) — The operating pressure the defect capacity is checked against
• Pipeline Dent Depth Screening Calculator (Plain Dents, %OD) — A dent containing metal loss needs both assessments, and neither alone
• Pipeline Hydrostatic Test Pressure Calculator (ASME B31.4 / B31.8) — Re-testing a corroded line: assess remaining strength before setting the test